It has been recognized from ancient times that fishes and other aquatic animals living in the sea and water produce noises and in some areas fishermen have located certain species of fish by listening for their characteristic sounds. Most detail of information on the production of sound by fishes has been gathered in the past few decades as the importance of underwater sound to naval operations and fisheries has become apparent. See Knudsen et al. Journal of Marine Research, Vol. 7, No. 43, 1948, pgs. 419-429 which is incorporated herein by reference. Also, the instrumentation necessary for research on sound has become more available in recent years.
Water is a good medium for the transmission of sound. Sounds can be carried long distances in water, being reflected off the bottom, the surface and density layers caused by temperature or salinity differences.
Most sounds made "on purpose" by fishes are the results of stridulation or the vibration of the gas bladder. Of course, the gas bladder can be involved in other ways, and may be brought into play to amplify or otherwise modify the stridulatory sounds. Grinding, snapping, or rubbing teeth together is the most common type of stridulation among fishes. Most gas bladder sounds arrive because of vibrations set up by some special means. Stridulation can be enhanced by the resonance of the gas bladder, but many noise makers are equipped with muscles that vibrate the gas bladder directly or indirectly.
Noises made by fishes have been described by a great variety of terms. Sounds of schools of fish swimming have been called rustles or roars. Stridulation produces sounds reminiscent of clicks, rasps, scratches, etc. when not aided by the gas bladder, and croaks, grunts, and knocks when the gas bladder acts as a resonator. Stridulatory sounds can range from 100 to 8000 hertz (Hz) although the gas bladder aided sounds are generally well below 1000 Hz and the unaided stridulations produce frequency usually in the 1000-4000 Hz range.
Sounds made by vibration of the gas bladder have been described as hoots, boops, grunts, yelps, and croaks. Gas bladder sounds are harmonic and usually of low frequency from 40 to 250 CPS with a great majority in the 75 to 100 Hz range.
Method and apparatus for locating and classifying and identifying underwater marine animals by means of the sounds they make are disclosed in Anderson U.S. Pat. No. 4,509,151 which is incorporated herein by reference. In Anderson, an elongated submersible array is adapted to be towed 300-400 feet behind a ship in various directions and at depths from 78-169 feet, depending on speed. The array is made up of subarrays of multiple hydrophones which are arranged along the length of each subarray in groups. By changing the combination of groups, the frequency response and directional sensitivity of the array is varied and the outputs are selectively analyzed both visually and audibly for characteristic fish sounds. There are many drawbacks to systems of the type disclosed in this Anderson patent. In order to give orientation to the fish, Anderson must move to work and requires a length which is at least a substantial fraction of the first acoustic wavelength and must be towed at a predetermined depth below the water surface with different sets of hydrophones being connected to different sets of filters in a very complex system. It requires a longitudinally elastic vibration isolation module which is coupled to the forward end of the acoustic array and this vibratory module has a predetermined buoyancy at a predetermined depth in the water, and rope drogue to put tension on the entire assembly.